1. Field of Invention
This invention relates to an exhaust-gases-purifying catalyst for purifying such toxic substances as hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx) contained in exhaust gases from an automotive engine or other internal combustion engines or boilers, a method of producing this catalyst and a method of purifying exhaust gases by using this catalyst.
2. Description of Related Art
As catalysts for purifying exhaust gases from automotive engines, catalysts have been widely used in which such a noble metal as platinum (Pt), rhodium (Rh) and palladium (Pd) is loaded on a support formed of such a porous oxide as alumina (Al2O3). For example, a 3-way catalyst is formed by loading Pt and/or Rh on such a porous oxide as xcex3-Al2O3, and oxidizes HC and CO in the exhaust gases into innocuous entities and at the same time reduces NOx in the exhaust gases into innocuous entities. Pt, which is especially active, is mainly used as a noble metal.
Regarding the production of an exhaust-gases-purifying catalyst like this three-way catalyst, a porous oxide support powder or a porous oxide support coated on a honeycomb-shaped supporting base material is brought in contact with an aqueous solution of a noble metal chloride, a noble metal nitro complex, or a noble metal ammonium complex, and then dried and calcined. Thus, a noble metal is loaded. The noble metal loaded by this method is highly dispersed on an atomic level, and the resulting catalyst attains an extremely high catalytic activity.
On the other hand, carbon dioxide (CO2) in the exhaust gases from the internal combustion engines of automobiles, etc. has recently become a problem in view of global environmental conservation. What we call xe2x80x9clean burnxe2x80x9d, i.e., burning lean fuel in oxygen-excessive atmospheres is desired as a solution to this problem. The lean burn improves fuel consumption, and as a result the amount of fuel consumed is decreased and CO2, which is a combustion exhaust gas, can be suppressed from generating.
In this respect, the conventional 3-way catalysts aim to oxidize CO and HC and reduce NOx simultaneously into innocuous entities when the air-fuel ratio is at the ideal air-fuel ratio, i.e., at the stoichiometric point, and cannot exhibit sufficient reduction and removal of NOx in the exhaust gases in oxygen-excessive atmospheres at the time of lean burn. Hence, it has been desired to develop catalysts which are capable of purifying NOx adequately even in oxygen-excessive atmospheres.
Under these circumstances, the applicants et al of the present invention have proposed an exhaust-gases-purifying catalyst in which Pt and an NOx storage component such as Ba are loaded on a porous support formed of alumina, etc., for example, in Japanese Unexamined Patent Publication (KOKAI) No. 5-317,625. By using this catalyst for purifying exhaust gases and controlling the air-fuel ratio to change pulsatorily from the fuel-lean side to the stoichiometric point or the fuel-rich side, namely, to perform xe2x80x9cfuel-rich spikesxe2x80x9d, NOx are adsorbed and stored by the NOx storage component on the fuel-lean side and the adsorbed NOx are released and react with reducing components such as HC and CO into innocuous entities at the stoichiometric point or on the fuel-rich side. Thus, NOx can be purified efficiently even in the case of lean burn.
In a method of producing an NOx storage and reduction type catalyst like this, a slurry including such a porous oxide as alumina and a binder is prepared and this slurry is coated on a honeycomb-shaped supporting base material formed of cordierite or a metal and calcined, thereby forming a coating layer. Next, the supporting base material having the coating layer thereon is immersed in a solution of a noble metal compound, thereby loading a noble metal, and then immersed in a solution of an NOx storage component, thereby loading the NOx storage component. Also known is another production method comprising preparing a slurry from a support powder in which an NOx storage component and a noble metal are loaded on alumina, etc., coating this slurry on a honeycomb supporting base material and calcining the material.
However, although the conventional 3-way catalysts and the NOx storage and reduction type catalysts exhibit superb catalytic activity in the initial stage of its use, they suffer from a drawback in that the catalytic activity gradually deteriorates as time passes. This degradation is particularly remarkable with the NOx storage and reduction type catalysts for purifying the exhaust gases from lean-burn engines. Our studies so far have clarified that the catalytic activity degradation with the passage of time is caused by the fact that since grains of noble metals, particularly Pt grow in fuel-lean oxygen-excessive atmospheres at high temperatures, the surface area of the noble metals decreases and consequently catalytic-activity sites decrease.
On the other hand, exhaust gases contain SO2, which is produced from a sulfur component of a fuel, and the SO2 is oxidized and react with an NOx storage component on a catalyst, thereby producing sulfates. The sulfates do not decompose and are stable around exhaust gas temperatures. Accordingly, there arises a problem that the NOx storage ability of the NOx storage component gradually disappears and the NOx purifying ability gradually deteriorates. This phenomenon is called sulfur poisoning of the NOx storage component.
The present invention has been developed in view of the aforementioned circumstances. It is a primary object of the present invention to suppress a decrease in catalytic activity by suppressing grain growth of a noble metal so as to improve the durability of 3-way catalytic activity and NOx conversion activity and provide an exhaust-gases-purifying catalyst with improved durability.
It is another object of the present invention to suppress sulfur poisoning of an NOx storage component in NOx storage and reduction type catalysts.
A catalyst for purifying exhaust gases recited in claim 2, which dissolves the above problems, is characterized in being produced by loading on a porous oxide support a noble metal composite colloid composed of a plurality of noble metals, and an NOx storage component selected from alkali metals, alkaline-earth metals and rare-earth elements.
A catalyst for purifying exhaust gases recited in claim 3 is characterized in being produced by loading a noble metal composite colloid composed of a plurality of noble metals on the surface of a porous oxide support, and loading an atomic-state noble metal in micro pores of the porous oxide support.
A catalyst for purifying exhaust gases recited in claim 4 is characterized in being produced by loading on the surface of a porous oxide support a noble metal composite colloid composed of a plurality of noble metals and an NOx storage component selected from alkali metals, alkaline-earth metals and rare-earth elements, and loading in micro pores of the porous oxide support a noble metal in the atomic state and the NOx storage component.
An exhaust-gases-purifying catalyst production method recited in claim 6, which is suitable to produce the exhaust-gases-purifying catalysts recited in claims 3 and 4, is characterized in comprising:
a separation step of taking a noble metal composite colloid out of a polymer-protected noble metal composite colloid which is composed of a plurality of noble metals and protected by a polymeric material;
a solution preparation step of dispersing the noble metal composite colloid in water by using a surfactant so as to prepare a surfactant-protected noble metal composite colloidal solution;
a noble-metal-colloid-loading step of bringing a porous oxide support in contact with the surfactant-protected noble metal composite colloidal solution so as to load a surfactant-protected noble metal composite colloid on the porous oxide support; and
an atomic-state-noble-metal-loading step of bringing the porous oxide support in contact with an aqueous solution of pyromellitic acid and a noble metal compound so as to load an atomic-state noble metal on the porous oxide support.
A method of purifying exhaust gases recited in claim 7 is characterized in placing, in exhaust gases in oxygen-excessive atmospheres, a catalyst produced by loading on a porous oxide support a noble metal composite colloid composed of a plurality of noble metals and an NOx storage component selected from alkali metals, alkaline-earth metals and rare-earth elements so as to adsorb NOx in the exhaust gases on the NOx storage component, and changing the exhaust gas atmospheres to the stoichiometric point or on the fuel-rich side so as to release the NOx from the NOx storage component and reduce the NOx.